In the production of large scale integrated semiconductor chips, ever more stringent requirements made of the fabrication installations and production processes used for the production of the semiconductor chips occur in particular by virtue of the ever advancing miniaturization of the structures on the semiconductor chip. One problem which occurs with the rising miniaturization of the large scale integrated semiconductor chips is the limitation of the miniaturization by the resolution capability of the lithography technology used which is employed for patterning the semiconductor chips of a wafer.
As an introduction, a lithography device 600 that can be used for patterning a wafer 601 is illustrated schematically in a simplified manner in FIG. 6. The lithography device 600 has an illumination unit 602 and a lens 603. The wafer 601 is patterned by being exposed using a mask 604 or reticle. For this purpose, a structure formed on the mask is imaged by means of laser light 605 and the illumination unit through the lens 603 onto the wafer 601, that is to say that the wafer 601 is exposed and a patterning of the wafer is thus possible.
Various methods for carrying out the lithography are known in the prior art. One is the use of a so-called “stepper”. When using such a stepper, an entire mask used is transferred all at once within a single exposure step onto a first exposure field of the wafer. Afterward, the wafer is moved on and the next exposure field of the wafer is exposed.
Another method which is used in lithography is one which is carried out by means of a so-called “scanner”. In the case of a scanner, the entire structure of a mask is not imaged onto a first exposure field of the wafer in one step, rather only a narrow strip of the mask is ever imaged all at once onto an exposure field of the wafer. For this purpose, a so-called exposure slot is used, which only ever illuminates a narrow strip of the mask and through which the mask is moved. During the exposure of an exposure field, the entire field gradually moves through the exposure slot. The mask is clearly scanned by means of this exposure slot. During the imaging of the mask onto a field of the wafer, both the mask and the wafer are moved. In this case, the movement of the wafer and of the mask generally takes place in opposite directions. To put it clearly, the mask is scanned by means of the exposure slot. In this case, every point on the mask is exposed during the movement through the movement slot with a plurality of laser flashes (pulses) onto the wafer.
The resolution of a lithography technology is given by equation (1):
  R  =            k      1        ·          λ              n        ·                  sin          ⁡                      (            θ            )                              
where: R is the resolution,
k1 is a process-dependent factor,
λ is the vacuum wavelength of the beam used for the lithography, and
n·sin(θ) is the so-called numerical aperture, where n is the refractive index of the medium in which the lithography is carried out, and θ is the aperture angle of the lens.
The process-dependent factor k1 has a value of greater than 0.25 for physical reasons. Clearly, k1 is greater than 0.25 in order to ensure that a uniform pattern of lines and interspaces, that is to say an alternation of bright and dark, can be imaged and is still discernible as such a pattern. In lithography, the wavelength is currently still limited to wavelengths of more than approximately 150 nm, since no materials which are transparent to light having a shorter wavelength are known to date.
It emerges from these boundary conditions that in order to increase the resolution capability, which increase is necessary for a lithography for the patterning of small structures, it is scarcely possible to make a change to k1 or to λ. Consequently, the only factor that remains is n·sin(θ), the so-called numerical aperture of the device, which is also designated as NA. In this case, it must be taken into consideration that sin(θ)≦1 holds true for mathematical reasons. Clearly, θ specifies the aperture angle at which light can enter into an imaging element (lens) in order that it also leaves the imaging element again without being subjected to total reflection, and is therefore a measure of the light intensity entering into the imaging element and the resolution capability of the lithography device.
Lithographic methods in semiconductor production have usually been carried out by means of air as the immersion medium, that is to say as the medium situated between the imaging element and the substrate. A refractive index of n≈1 thus results. If the lithographic method is carried out with a medium different than air, that is to say if a so-called immersion lithography is carried out, then the resolution capability can be improved by a factor which is equal to the refractive index of the immersion medium. In the case of such an immersion method, a liquid having a refractive index of n>1 is introduced into an interspace between an imaging element, that is to say e.g. a lens, and a lithography device.
The use of an immersion medium makes it possible to have the effect that additional light contributes to the light intensity of the imaging element. Light which is incident in the imaging element at an angle which is too large to still contribute to the light intensity of the imaging element given an immersion medium of air, that is to say would be subjected to total reflection, can still contribute to the light intensity given the use of an immersion medium with a higher refractive index than n=1. As a result of this, it is possible to obtain a better resolution, or the depth of focus of the imaging can be increased for the same resolution.
One disadvantage of immersion lithography, however, is that the immersion medium absorbs part of the light which is used for the exposure of the wafer. The immersion medium is heated as a result of the absorption. The heating of the immersion medium in turn leads to a change in the refractive index of the immersion medium. For water, there are estimations for the change in the refractive index with the temperature T which amount to approximately dn/dT=10−4K−1 for a wavelength of λ=193 nm.
This in turn leads to a slight change in the distance between the imaging element and the wafer, at which distance the best focusing can be obtained, that is to say that the imaging is sharpest or, to put it another way, the resolution takes up the smallest value. The change in the temperature and hence in the refractive index of the liquid also leads to a reduction of the depth of focus (DoF) of the imaging. In a lithography method, the depth of focus of the projected image, that is to say the image of the mask, is thereby reduced, thus resulting in a reduction of a processing window for the lithography method, that is to say which fluctuation range the lithography parameters are permitted to have.
One approach to solving this problem lies in controlling the temperature of the immersion liquid. That is to say that it is attempted to keep the temperature as far as possible constant and to stabilize it within a small temperature interval. However, this has to be effected very exactly. Such exact temperature control is costly and can only be achieved with difficulty. Focal changes that remain furthermore have an adverse influence on the depth of focus of the imaging and on the resolution of the lithography method.
In order approximately to specify the order of magnitude of how exactly the temperature is to be complied with and how great the influence is of a change in temperature that remains, this will be estimated on the basis of an example. For a wavelength of λ=193 nm, a refractive index of n=1.47 (deionized water), a sin(θ)=0.75 and a working distance, that is to say a distance between the imaging element and the wafer surface to be patterned, of D=1 mm, δn<6·10−7 has to be complied with if a change in the distance of sharp imaging of ΔD<1 nm is intended to be complied with, where δn is the change in the refractive index. From δn<6·10−7 and the estimation of dn/dT=10−4K−1 already discussed above, it is possible to calculate on the basis of equation (2)
      Δ    ⁢                  ⁢    D    =                    D        ·        δ            ⁢                          ⁢      n                      n        ·                  cos          2                    ⁢      θ      
how exactly the temperature must be controlled and regulated. A required accuracy of 6 mK results. This accuracy of the temperature control can be complied with only with difficulty, as a result of which the use of immersion lithography in the patterning of semiconductor elements is greatly impeded and made greatly difficult.
U.S. Pat. No. 6,191,429 to Suwa discloses a focusing device which has an objective system for optically producing a workpiece, for forming a desired pattern on a surface of a workpiece or for inspecting a pattern on a workpiece, and which is used to set the focus state between the surface of the workpiece and the objective system.
U.S. Pat. No. 6,586,160 to Ho, et al. discloses a scanning exposure system which provides light which comprises items of pattern information which are intended to be transferred onto a wafer, and thus patterns a photoresist layer on the semiconductor wafer.
Japanese Patent No. JP10303114 discloses an immersion lithography device, a working distance between the device and a workpiece satisfying a relation which takes account of the temperature coefficient of the refractive index of the immersion fluid and the temperature.
U.S. Pat. No.6,509,952 to Govil, et al. discloses that linewidth control parameters vary within a pattern as a consequence of properties of a lithography device, and that these variations can be compensated for by means of linewidth offset coefficients.